Preheat your oven to 350°F (175°C) and grease a 9 x 13-inch baking pan.

In a large mixing bowl, combine the dry ingredients: granulated sugar, dark cocoa powder, Bob's Red Mill gluten-free flour, baking soda, baking powder, and salt. Whisk until fully blended.

In a separate bowl, whisk together the wet ingredients: vegetable oil, eggs, and buttermilk until smooth and slightly frothy.

Pour the wet mixture into the dry ingredients and fold together gently. Stir in brewed coffee, creating a velvety batter.

Pour the batter into the greased pan and bake for approximately 30 minutes or until a toothpick inserted comes out clean.

Remove from the oven and let the cake cool in the pan for about 10 minutes, then transfer to a wire rack to cool completely.

For the frosting, beat the unsalted butter until creamy, then add cocoa powder, powdered sugar, vanilla, and salt, mixing until fluffy.

Spread the chocolate buttercream evenly over the cooled cake.